EP0695630A1 - Biaxially orientated polyolefin laminate with low sealing-temperature, method of making it and its use - Google Patents

Abstract

The novel feature in a multilayer polyolefin film with a polypropylene-contg. base layer is that the top layer contains at least 50 wt.% of a low crystalline C3/C4 olefin resin mixt. (I) and is modified to have a seal initiating temp. of 85-110 deg.C by corona treatment and/or addn. of a propylene polymer. Claimed prodn. is by co-extrusion using draw-off rolls at 10-90 deg.C, followed by biaxially stretching at 4-7:1 (longitudinal) and 6-11:1 (transverse), heat fixing and opt. also corona treatment.

Description

The invention relates to a polyolefin multilayer film comprising a base layer which contains polypropylene and at least one cover layer. The invention further relates to the use of this film for wrapping packaging with at least three superimposed film layers on the end faces.

Packaging machines that run ever faster, as well as sensitive filling goods (food, chocolate, pastries) require foils with a low sealing start temperature in combination with optimum machine running behavior. Films are known in the prior art which have sealing initiation temperatures of around 100 ° C., in some cases even down to 74 ° C. However, these films have serious disadvantages in the production or in their use.

EP-A-0 114 312 and EP-A-0 114 311 describe transparent or opaque polypropylene films with a seal initiation temperature of below 100 ° C. and with good machinability. These films have a top layer of a mixture of C₂ / C₃ / C₄ terpolymer and C₃ / C₄ copolymer and a low molecular weight resin, polypropylene homopolymer and polydiorganosiloxane. According to this teaching, the large sealing area of the film and the low sealing initiation temperature are achieved by the olefin resin composition made of co- and terpolymer. The minimum seal starting temperature is 90 ° C. The incorporation of hydrocarbon resins in the top layer leads to resin deposits on the stretching rollers during manufacture. These deposits lead to streaking. The rollers need to be cleaned frequently.

EP-A-0 480 282 describes a biaxially oriented polyolefin multilayer film with a low sealing on both sides, the base layer of which contains a peroxide-degraded propylene homopolymer and the top layers of which are mixtures of C₂ / C₃ / C₄ terpolymer and C₂ / C₃ copolymer and a combination of SiO₂ and Contain polydialkylsiloxane. The polydialkylsiloxane is highly viscous and is added in the form of a masterbatch. The sealing start temperature is 88 ° C.

The as yet unpublished European patent application No. 94101786.5 describes multilayer films with at least one low-sealing cover layer. The top layer contains a low crystalline C₃ / C₄ olefin resin composition and has a sealing start temperature of less than 84 ° C. Films with this top layer tend to stick strongly to themselves and to hot machine parts such as B. Stretch Rollers. Furthermore, the packaging made therefrom, especially when stored in the sun at summer temperatures such. B. in a cigarette machine, easy to block and are therefore unsuitable for this use.

Furthermore, sealable outer layers of C₂ / C₃ copolymers and of C₂ / C₃ / C₄ terpolymers are known in the prior art. These foils all have sealing start temperatures of over 100 ° C.

Also known are cigarette packaging, which preferably consist of multilayer opaque films as described in EP-A-0 582 953. Conventional propylene polymers and mixtures thereof are described here as cover layer materials. These top layer raw materials used according to the prior art have the disadvantages described above of the high sealing starting temperature, the roller deposits or the tendency to stick and must be optimized. Furthermore, it has been found that the sealing seam of the film on the end faces of the cigarette packaging is defective. In particular those places The sealing seam, which is formed from more than three layers of foil lying one above the other, is particularly critical since the necessary heat does not pass through all of these layers in the sealing time available.

The invention is based on the object of providing a multilayer film which has good sealing properties and can also be sealed at high cycle speeds without problems even in the case of multilayer sealed seams, in order to also be usable for heat-sensitive packaging goods and on high-speed packaging machines. The film should not tend to block under the usual storage conditions. Furthermore, the film should not cause deposits during production, e.g. B. on stretching rollers. A low level of haze is required for transparent embodiments of the film. In a preferred embodiment, the low-sealing side should be printable.

According to the invention this object is achieved by a film of the type mentioned, the characteristic features of which are that the cover layer contains at least 50 wt .-% of a low crystalline C₃ / C₄ olefin resin composition, which by corona treatment and / or by the addition of a propylene polymer is modified, and the top layer has a seal starting temperature of 85 to 110 ° C.

Depending on the intended area of use, the respective embodiment of the film can be transparent, matt, opaque, white or white-opaque.

The film according to the invention comprises at least one core layer and the cover layer essential to the invention, the core layer essentially containing a propylene polymer or a polypropylene mixture.

The propylene polymer contains predominantly (at least 90%) propylene and has a melting point of 140 ° C. or higher, preferably 150 to 170 ° C. Isotactic homopolypropylene with an n-heptane soluble content of 6% by weight or less, based on the isotactic homopolypropylene, copolymers of ethylene and propylene with an ethylene content of 10% by weight or less, copolymers of propylene with C₄-C₈-α- Olefins with an α-olefin content of 10% by weight or less are preferred propylene polymers for the core layer, with isotactic homopolypropylene being particularly preferred. The percentages by weight refer to the respective copolymer. The propylene polymer of the core layer generally has a melt flow index of 0.5 g / 10 min to 8 g / 10 min, preferably 2 g / 10 min to 5 g / 10 min, at 230 ° C and a force of 21.6 N ( DIN 53 735). A mixture of the propylene homo- and / or copolymers and / or other polyolefins, in particular having 2 to 6 carbon atoms, is also suitable, the mixture being at least 50% by weight, in particular at least 75% by weight, Contains propylene polymer. Suitable other polyolefins in the polymer mixture are polyethylenes, in particular HDPE, LDPE and LLDPE, the proportion of these polyolefins in each case not exceeding 15% by weight, based on the polymer mixture.

MFI₁ =

Schmelzflußindex des Polypropylenpolymeren vor dem Zusatz des organischen Peroxids

MFI₂ =

Schmelzflußindex des peroxidisch abgebauten Polypropylenpolymeren

Im allgemeinen liegt der Abbaufaktor A des eingesetzten Polypropylenpolymeren in einem Bereich von 3 bis 15, vorzugsweise 6 bis 10.If necessary, the polypropylene polymer used in the core layer can be partially degraded by the addition of organic peroxides. A measure of the degree of degradation of the polymer is the so-called degradation factor A, which indicates the relative change in the melt flow index according to DIN 53 735 of the polypropylene, based on the starting polymer.

MFI₁ =

Melt flow index of the polypropylene polymer before the addition of the organic peroxide

MFI₂ =

Melt flow index of the peroxidically degraded polypropylene polymer

In general, the degradation factor A of the polypropylene polymer used is in a range from 3 to 15, preferably 6 to 10.

Dialkyl peroxides are particularly preferred as organic peroxides, an alkyl radical being understood to mean the customary saturated straight-chain or branched lower alkyl radicals having up to six carbon atoms. In particular, 2,5-dimethyl-2,5-di (t-butylperoxy) hexane or di-t-butyl peroxide are preferred.

Opaque, white or white-opaque embodiments of the films additionally contain fillers in an amount of 5 to 20% by weight, based on the total weight of the film, in the core layer and / or in further layers.

The fillers of the base layer are the usual inorganic and / or organic materials which are incompatible with polypropylene. Inorganic fillers are preferred, in particular aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates such as aluminum silicate (kaolin clay) and magnesium silicate (talc), silicon dioxide and / or titanium dioxide, among which calcium carbonate, silicon dioxide, titanium dioxide or mixtures thereof are preferably used. Calcium carbonate (chalk) is particularly preferred. Suitable organic fillers are the customarily used polymers which are incompatible with the polymer of the base layer, in particular those such as polyesters, polystyrenes, polyamides, halogenated organic polymers, polyesters such as polybutylene terephthalates being preferred. In addition, cycloolefin polymers are also particularly suitable as fillers. The amount of filler in the base layer, in particular that of CaCO₃, according to the invention is 1 to 20% by weight, preferably 3 to 15% by weight, based on the total weight of the film. The average particle size of the fillers, in particular that of CaCO₃, is in the range from 1 to 5 µm, preferably in the range from 1.5 to 4 µm.

Even with a relatively low filler content of 3 to 13% by weight, the density of the opaque embodiments is less than 0.8 g / cm 3. In general, the density of the opaque film according to the invention is in the desirable range of 0.4 to 0.75 g / cm³.

In addition to the preferred calcium carbonate, titanium dioxide, which is optionally used in combination with CaCO₃ in the base layer, is suitable as a further core layer additive. In a preferred embodiment, the titanium dioxide particles consist of at least 95% by weight of rutile. The titanium dioxide particles are preferably used with a coating of inorganic oxides, as is usually used as a coating for TiO₂ white pigment in papers or paints to improve lightfastness. The particularly suitable inorganic oxides include the oxides of aluminum, silicon, zinc or magnesium or mixtures of two or more of these compounds. They are made from water-soluble compounds, e.g. As alkali, especially sodium aluminate, aluminum hydroxide, aluminum sulfate, aluminum nitrate, sodium silicate or silica, precipitated in the aqueous suspension. TiO₂ particles with a coating are, for. B. in EP-A-0 078 633 and EP-A-0 044 515.

The coating may also contain organic compounds with polar and non-polar groups. Preferred organic compounds are alkanols and fatty acids with 8 to 30 carbon atoms in the alkyl group, in particular fatty acids and primary n-alkanols with 12 to 24 carbon atoms, and also polydiorganosiloxanes and / or polyorganohydrogensiloxanes such as polydimethylsiloxane and polymethylhydrogensiloxane.

The coating on the TiO₂ particles usually consists of 1 to 12 g, in particular 2 to 6 g, of inorganic oxides, optionally 0.5 to 3 g, in particular 0.7 to 1.5 g, of organic compounds, based on 100 g TiO₂ particles, contain. The coating is applied to the particles in aqueous suspension. It has proven to be particularly advantageous if the TiO₂ particles are coated with Al₂O₃ or with Al₂O₃ and polydimethylsiloxane.

The proportion of titanium dioxide in the core layer - if present - is 1 to 9% by weight, preferably 3 to 5% by weight, based on the total weight of the film. The average particle size is very small and is preferably 0.15 to 0.40 μm, the particle size stated relating to the coated particles.

TiO₂ is then preferably used in addition to CaCO₃ in the base layer if a white-opaque appearance of the film is desired. Films with a filler-containing base layer usually have a cloudy, so-called opaque appearance, which is caused by scattering processes at the vacuoles of the base layer. To be distinguished from this is a white appearance, which is achieved by coloring pigments, preferably TiO₂.

Furthermore, the core layer may optionally contain a low molecular weight resin, the proportion of which is 1 to 30% by weight, preferably 2 to 10% by weight. The softening point of the resin is between 130 and 180 ° C (measured according to DIN 1995-U4, corresponds to ASTM E-28), preferably between 140 and 160 ° C. Among the numerous low molecular weight resins, the hydrocarbon resins are preferred, namely in the form of petroleum resins (petroleum resins), styrene resins, cyclopentadiene resins and terpene resins (these resins are described in Ullmanns Encyklopadie der techn. Chemie, 4th edition, volume 12, pages 525 to 555) ). Suitable petroleum resins are described in numerous documents, such as EP-A-0 180 087, to which reference is expressly made here.

Furthermore, it is preferred to add antistatic agents such as, for example, tertiary aliphatic amines or glycerol monostearate to the base layer. The type and amount of the antistatic agent are described in detail below.

In addition to the core layer, the multilayer film according to the invention comprises at least one low-sealing cover layer which contains a low-crystalline C₃ / C₄ olefin resin composition. In general, the cover layer contains at least 50% by weight, preferably 60 to 100% by weight, in particular 70 to 95% by weight, based in each case on the weight of the cover layer, of the olefin resin composition. It is essential to the invention that this cover layer is modified by suitable measures so that the sealing start temperature is in the range from 85 to 110.degree. The C₃ / C₄ olefin resin composition has, without these measures, ie in its pure form, a seal starting temperature of below 84 ° C. It has been found that the corona or flame treatment of the C₃ / C₄-olefin resin-containing top layer increases the sealing starting temperature to a range from 85 to 110 ° C and at the same time surprisingly improves other properties of the film. As an alternative to the corona treatment, the C₃ / C₄ olefin resin composition can also be modified by blending with other propylene polymers, this blending likewise having to increase the sealing start temperature to 85 to 110 ° C. The low crystalline C₃ / C₄ olefin resin composition can be characterized by the following information: Melting temperature: <130 ° C, preferably 105 to 120 ° C Recrystallization temperature: <80 ° C, preferably 55 to 80 ° C Enthalpy of fusion: <67 J / g, preferably 50 to 60 J / g C₃ content: 60 to 85% by weight, preferably 70 to 80% by weight, in each case based on the weight of the copolymer, or 60 to 85 mol%, preferably 70 to 80 mol%, in each case based on the copolymer C₄ content: 15 to 40% by weight, preferably 20 to 30% by weight, in each case based on the weight of the copolymer, or 15 to 40 mol%, preferably 20 to 30 mol%, in each case based on the copolymer

In particular, an olefin resin composition is preferred whose melting temperature is 110.6 ° C, whose recrystallization temperature is 60 ° C and whose melting enthalpy is 56.7 J / g. The sealing start temperature of a cover layer made of this pure component is in the range from 65 to 80 ° C.

The cover layer contains the propylene polymers suitable for modification in an amount of at most 50% by weight, preferably more than 0 to 40% by weight, in particular 5 to 30% by weight, in each case based on the weight of the cover layer. The following polymers are particularly suitable as propylene polymers for modification:

Propylene homopolymer or copolymer of

Ethylene and propylene or
Ethylene and butylene-1 or
Propylene and butylene-1 or

Terpolymer of

Ethylene and propylene and butylene-1 or
a mixture of two or more of the homo-, co- and
Terpolymers or
a blend of two or more of the homopolymers, copolymers and terpolymers mentioned, optionally mixed with one or more of the homopolymers, copolymers and terpolymers mentioned,
wherein in particular propylene homopolymer or
statistical ethylene-propylene copolymers with
an ethylene content of 1 to 10% by weight, preferably 2.5 to 8% by weight, or
statistical propylene-butylene-1 copolymers with
a butylene content of 2 to 25% by weight, preferably 4 to 20% by weight,
each based on the total weight of the copolymer, or statistical ethylene-propylene-butylene-1-terpolymers
an ethylene content of 1 to 10% by weight, preferably 2 to 6% by weight, and
a butylene-1 content of 2 to 20% by weight, preferably 4 to 20% by weight,
each based on the total weight of the terpolymer, or
a blend of an ethylene-propylene-butylene-1 terpolymer and a propylene-butylene-1 copolymer
with an ethylene content of 0.1 to 7% by weight
and a propylene content of 50 to 90% by weight
and a butylene-1 content of 10 to 40% by weight,
each based on the total weight of the polymer blend, are preferred.

Polymers such as ®Tafmer XR 107L from Mitsui and ®PKS 309 from Solvay are particularly preferred for modification.

If necessary, the cover layer modified by mixing can also be additionally corona or flame treated.

In a preferred embodiment, the top layer according to the invention contains an antiblocking agent. The type and amount of antiblocking agents are described below.

The multilayer film has at least two layers and always comprises the core layer and at least one top layer according to the invention as essential layers. Depending on its intended use, the film can have a further cover layer and / or further intermediate layers. Covering layers on both sides can in principle be the same or different according to their structure, their composition and their thickness. H. the second top layer can contain both the same modified, low crystalline olefin resin composition as described above or other known sealable propylene polymers such as C₂ / C₃ copolymers, C₂ / C₃ / C₄ terpolymers or mixtures thereof or non-sealable homopolymers. An embodiment is preferred in which the second cover layer cannot be printed on and has a minimum seal starting temperature which is above that of the cover layer according to the invention.

Furthermore, the multilayer film according to the invention can have intermediate layers on one and / or both sides. Interlayers are particularly preferred for opaque embodiments and then preferably contain the above-described TiO₂ as an additive.

The thickness of the low-sealing, modified top layer according to the invention is greater than 0.3 μm and is generally 0.4 to 5 μm. The cover layer thickness is preferably in the range from 0.6 to 3 μm, in particular in the range from 0.8 to 1.5 μm. The total thickness of the film depends on its intended use and can vary within wide limits. In general, the total thickness is in the range from 5 to 150 μm, preferably in the range from 10 to 100 μm, in particular in the range from 15 to 70 μm. Opaque embodiments are in the range from 5 to 150 μm, preferably in the range from 40 to 100 μm, in particular in the range from 60 to 90 μm.

In order to further improve certain properties of the polypropylene film according to the invention, the core layer and the top layer (s) may contain further additives, which do not impair the essential film properties, in an effective amount in each case, preferably lubricants and / or stabilizers and / or neutralizing agents which are combined with the Polymers of the core layer and the top layer / s are compatible. All quantities in the following embodiment in percent by weight (% by weight) relate to the layer or layers to which the additive can be added.

Lubricants are higher aliphatic acid amides, higher aliphatic acid esters, waxes and metal soaps as well as polydimethylsiloxanes. The effective amount of lubricant is in the range of 0.1 to 3% by weight. The addition of higher aliphatic acid amides in the range from 0.15 to 0.25% by weight in the base layer and / or the top layers is particularly suitable. A particularly suitable aliphatic acid amide is erucic acid amide.

The addition of polydimethylsiloxanes is preferred in the range from 0.05 to 2.0% by weight, in particular in the range from 0.07 to 0.2% by weight, in particular polydimethylsiloxanes with a viscosity of 10,000 to 1,000,000 mm² / s.

The usual stabilizing compounds for ethylene, propylene and other α-olefin polymers can be used as stabilizers. The amount added is between 0.05 and 2% by weight. Phenolic stabilizers, alkali / alkaline earth stearates and / or alkali / alkaline earth carbonates are particularly suitable. Phenolic stabilizers are preferred in an amount of 0.1 to 0.6% by weight, in particular 0.15 to 0.3% by weight, and with a molar mass of more than 500 g / mol. Pentaerythrityl tetrakis-3- (3,5-di-tertiary-butyl-4-hydroxyphenyl) propionate or 1,3,5-trimethyl-2,4,6-tris (3,5-di-tertiary-butyl-4-hydroxybenzyl) benzene are particularly advantageous.

Neutralizing agents are preferably calcium stearate and / or calcium carbonate with an average particle size of at most 0.7 µm, an absolute particle size of less than 10 µm and a specific surface area of at least 40 m² / g.

The invention further relates to a method for producing the multilayer film according to the invention by the coextrusion method known per se. In this process, the procedure is such that the melts corresponding to the individual layers of the film are co-extruded through a flat die, the film thus obtained is drawn off for solidification on one or more rollers, the film is then biaxially stretched (oriented), the biaxial stretched film is heat-set and optionally corona-treated on the surface layer provided for the corona treatment.

The biaxial stretching (orientation) is generally carried out in succession, with the successive biaxial stretching, in which stretching first lengthwise (in the machine direction) and then transversely (perpendicular to the machine direction) being preferred.

First, as is customary in the coextrusion process, the polymer or the polymer mixture of the individual layers is compressed and liquefied in an extruder, it being possible for the additives which may have been added to be present in the polymer. The melts are then simultaneously pressed through a flat die (slot die), and the pressed multilayer film is drawn off on one or more take-off rolls, as it cools and solidifies.

The film thus obtained is then stretched longitudinally and transversely to the direction of extrusion, which leads to an orientation of the molecular chains. In the longitudinal direction it is preferably 4: 1 to 7: 1 and in the transverse direction preferably 6: 1 to 11: 1. The longitudinal stretching is expediently carried out with the aid of two rollers running at different speeds in accordance with the desired stretching ratio, and the transverse stretching with the aid of a corresponding tenter frame.

The biaxial stretching of the film is followed by its heat setting (heat treatment), the film being held at a temperature of 120 to 170 ° C., preferably 140 to 160 ° C., for about 0.5 to 10 s. The film is then wound up in a conventional manner using a winding device.

It has proven to be particularly advantageous to keep the take-off roller or rollers, by means of which the pressed film is also cooled and solidified, at a temperature of 10 to 90 ° C., preferably 20 to 60 ° C.

In addition, the longitudinal stretching is advantageously carried out at a temperature of less than 150 ° C., preferably in the range from 110 to 140 ° C., and the transverse stretching is carried out at a temperature greater than 140 ° C., preferably at 145 to 180 ° C. The film is brought to the required stretching temperature using an air heating box.

After the biaxial stretching, the film for the modification of the C₃ / C₄ resin-containing cover layer on this surface is corona or flame treated. If the cover layer is modified by blending with other propylene polymers, the appropriate treatment may be dispensed with.

In the corona treatment, the procedure is such that the film is passed between two conductor elements serving as electrodes, such a high voltage, usually alternating voltage (approximately 10,000 V and 10,000 Hz), being applied between the electrodes that spray or corona discharges take place can. The air is above the spray or corona discharge the film surface ionizes and reacts with the molecules of the film surface, so that polar inclusions are formed in the essentially non-polar polymer matrix. The treatment intensities are in the usual range, with 38 to 45 mN / m being preferred.

In the flame treatment, the film is passed over a cooling roll, above which a gas burner is arranged. The film is pressed firmly onto the cooling roller by a nip roller. The gas flowing out of the burner is ignited and forms flames about 5 to 10 mm in size. The oxidizing part of the flame hits the surface of the film and increases the surface energy of the film. Here too, it is within the usual framework.

The film according to the invention is characterized by a low seal initiation temperature in the range from 85 to 110 ° C., preferably 85 to 95 ° C., a good seal strength being surprisingly achieved even when sealing or by means of several layers of the film. The film according to the invention is therefore particularly suitable for cigarette wrapping on high-speed packaging machines.

This type of packaging is characterized by the wrap on the front of the packaging. At this point, due to the multiple folding, at least three or even five layers of the film come to lie on one another. When sealing on these end faces, the heat required for sealing must penetrate these layers of film in a sufficiently short time. Surprisingly, the film according to the invention with the modified top layer can be used excellently for this application. This is particularly surprising in the case of the opaque embodiments of the film, which has particularly good heat insulation properties due to its high thickness and its vacuole-containing structure.

In addition, the modified top layer is characterized by good warm block values. The film can be easily manufactured and processed. It does not tend to stick to the rollers and does not stick to itself even at high temperatures.

The low seal starting temperature also opens up new areas of application, e.g. B. as a label film without additional adhesion promoter.

The invention is illustrated by the following examples:

example 1

A three-layer opaque film with a total thickness of 60 μm, a density of 0.75 g / cm 3 and an ABC layer structure was produced by coextrusion and subsequent stepwise orientation in the longitudinal and transverse directions, i. H. the base layer B is surrounded by a cover layer A and a cover layer C. Before being rolled up, the film was corona-treated on the A-side. The surface tension on this side was 39 to 40 mN / m due to the corona treatment.

All layers contained 0.12% by weight of pentaerythrithyl tetrakis 4- (3,5-di-tertiary-butyl-4-hydroxyphenyl) propionate (®Irganox 1010) and 0.06% by weight of calcium stearate as neutralizing agent.

The base layer B essentially consisted of a polypropylene homopolymer with an n-heptane-soluble fraction of 3.5% by weight and a melting point of 162 ° C. The melt flow index of the polypropylene homopolymer was 3.4 g / 10 min at 230 ° C and 21.6 N load (DIN 53 735). The base layer also contained 4.3% by weight of the chalk masterbatch ®MP 52355 from Multibase and 4% by weight of the TiO₂ batch ®P 8555 from Schulman.

The polyolefinic cover layer A consisted essentially of a mixture of 90% by weight of a random propylene-butene-1 copolymer (®Tafmer XR 110 T) with a propylene content of 74.3 mol% and a butene-1 content of 25.7 mol%, based on the copolymer, a melting temperature of 110.6 ° C and a recrystallization temperature of 60 ° C and 10% by weight of ethylene-propylene-butene terpolymer ®Tafmer XR 107. The top layer A contained 0.35% by weight of the antiblocking agent silicon dioxide with an average particle diameter of 4 μm. The cover layer thickness was 0.8 µm.

The polyolefinic cover layer C consisted essentially of ethylene-propylene-butene-1 terpolymers with a content of 3.5% by weight of ethylene, 88.5% by weight of propylene and 8% by weight of butene-1. The terpolymer contained 0.30 wt .-% silicon dioxide with an average particle diameter of 4 microns. The thickness of the cover layer was 0.8 µm.

Example 2

Example 1 was repeated. The polyolefinic cover layer A, which was corona-treated, consisted of 60% by weight of a random propylene-butene-1 copolymer with a content of 74.3 mol% of propylene and 25.7 mol% of butene-1 and 40% % By weight from ®Tafmer XR 107. The copolymer contained 0.35% by weight of a silicon dioxide with an average particle diameter of 4 μm. The thickness of the cover layer was 0.9 µm.

Comparative Example 1

Example 1 was repeated. The polyolefinic cover layer A consisted essentially of a propylene-butene-1 copolymer with a content of 74.3 mol% of propylene and 25.7 mol% of butene-1. The top layer contained 0.35% by weight of the antiblocking agent silicon dioxide with an average particle diameter of 4 μm. The cover layer thickness was 0.8 µm.

The properties of the films according to the examples and comparative examples are summarized in the table below.

The following measurement methods were used to characterize the raw materials and the foils:

Melt Flow Index (MFI)

DIN 53 735 at 21.6 N load and 230 ° C.

Melting point

DSC measurement, maximum of the melting curve, heating rate 20 ° C / min.

shine

The gloss was determined in accordance with DIN 67 530. The reflector value was measured as an optical parameter for the surface of a film. Based on the standards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at 60 ° or 20 °. A light beam hits the flat test surface at the set angle of incidence and is reflected or scattered by it. The light rays striking the photoelectronic receiver are displayed as a proportional electrical quantity. The measured value is dimensionless and must be specified with the angle of incidence.

Seal seam strength at 100 ° C

For the determination, two 15 mm wide strips of film were placed on top of one another and sealed at 100 ° C., a sealing time of 0.5 s and a sealing pressure of 10 mm² (device: Brugger type NDS, sealing jaw heated on one side). The seal seam strength was determined by the T-Peel method.

Determination of the seal starting temperature:

With the sealing device HSG / ET from Brugger, heat-sealed samples (sealing seam 20 mm x 100 mm) are produced by using a film at different temperatures with the help of two heated sealing jaws with a sealing pressure of 10 N / cm² and a sealing time of 0.5 s is sealed. Test strips 15 mm wide are cut from the sealed samples. The T-seal strength, i.e. H. the force required to separate the test strips is determined with a tensile testing machine at a pull-off speed of 200 mm / min, the sealing seam plane forming a right angle with the direction of pull. The seal start temperature (or minimum seal temperature) is the temperature at which a seal seam strength of at least 0.5 N / 15 mm is achieved.

Determination of the hot block behavior

To measure the hot-block behavior, two wooden blocks with the dimensions 72 mm x 41 mm x 13 mm, felt-glued on one side, are wrapped and sealed in the film to be measured. A weight of 200 g is placed on the wooden blocks facing each other with the felt pads and this structure is placed in a heating oven preheated to 70 ° C. and left there for 2 hours. The mixture is then cooled to room temperature (21 ° C.) for 30 minutes, the weight is removed from the wooden blocks and the upper block is pulled down from the lower block by means of mechanical equipment. The evaluation is carried out using 4 individual measurements, which are then used to determine a maximum push-off force (measured in N). The specification is met if none of the individual measurements is above 5 N.

Claims (16)

Polyolefinic multilayer film comprising a base layer which contains polypropylene and at least one top layer, characterized in that the top layer contains at least 50% by weight of a low-crystalline C₃ / C₄ olefin resin composition which is obtained by corona treatment and / or by the addition of a propylene polymer is modified, and the top layer has a seal starting temperature of 85 to 110 ° C. Multilayer film according to Claim 1, characterized in that the sealing initiation temperature of the cover layer is in the range from 85 to 95 ° C and the olefin resin composition has a melting temperature of less than 130 ° C, preferably 105 to 120 ° C. Multilayer film according to Claim 1 and / or 2, characterized in that the olefin resin composition has a recrystallization temperature of less than 80 ° C, preferably 55 to 70 ° C, and a melting enthalpy of <67 J / g, preferably 50 to 60 J / g. Multilayer film according to one or more of Claims 1 to 3, characterized in that the olefin resin composition has a propylene content of 70 to 80 mol%, preferably 75 mol%, and a butene-1 content of 20 to 30 mol%, preferably 25 Mol%, based in each case on the olefin resin composition. Multilayer film according to one or more of claims 1 to 4, characterized in that the cover layer comprises at least 50% by weight, preferably 50 to 95% by weight, in particular 60 to 90% by weight, in each case based on the weight of the cover layer containing C₃ / C₄ olefin resin composition. Multi-layer film according to one or more of claims 1 to 5, characterized in that it has three, four or five layers. Multi-layer film according to one or more of claims 1 to 6, characterized in that it is transparent. Multi-layer film according to one or more of claims 1 to 6, characterized in that it is opaque. Multilayer film according to one or more of Claims 1 to 8, characterized in that the base layer contains an antistatic, preferably tertiary aliphatic amine, and the top layer contains an antiblocking agent, preferably SiO₂, with a particle size of 4 to 6 µm. A process for producing the multilayer film according to claim 1, in which the melts corresponding to the individual layers of the film are coextruded through a flat die, the coextruded film is drawn off via a take-off roller, the temperature of which is between 10 and 90 ° C, and the film is stretched biaxially with a longitudinal stretch ratio of 4: 1 to 7: 1 and a transverse stretch ratio of 6: 1 to 11: 1, the biaxially stretched film is heat-set, optionally corona-treated and then wound up. Use of the multilayer film according to one or more of claims 1 to 9 as packaging film. Use of the multilayer film according to one or more of claims 1 to 9 as packaging film for single-material packaging. Use of the multilayer film according to one or more of claims 1 to 9 as packaging film for wrapping packages with at least three layers of film lying one above the other on the end faces. Use of the multilayer film according to one or more of claims 1 to 9 for the production of laminates, preferably laminates with paper, cardboard, metals, metallized plastic films and plastic films. Laminates containing a multilayer film according to one or more of claims 1 to 9. Packaging containing a multilayer film according to one or more of claims 1 to 9.